This invention relates generally to flowmeters of the vortex type, and more particularly to adapters for a flowmeter to reduce the effective area of fluid passing the vortex shedding body, thereby making it possible for the meter to measure flow rates below its normal operating range.
It is well known that under certain circumstances the presence of an obstacle in a flow conduit will give rise to periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed. These patterns are known as Karman vortex streets. The frequency at which vortices are shed in a Karman vortex street is a function of flow rate, this phenomenon being exploited to create a flowmeter. Flowmeters of the vortex-shedding type are disclosed in U.S. Pat. Nos. 3,116,639 and No. 3,572,117, among others.
U.S. Pat. No. 3,589,185 describes an improved form of vortex-type flowmeter wherein the signal derived from the fluidic oscillation is relatively strong and stable to afford a favorable signal-to-noise ratio, insuring accurate flowrate information over a broad range. In this meter, an obstacle assembly is mounted in the flow tube, the assembly being constituted by a block positioned across the tube with its longitudinal axis at right angles to the direction of fluid flow, a strip being similarly mounted across the conduit behind the block and being spaced therefrom to define a gap which serves to trap Karman vortices and to strengthen and stabilize the vortex street. This street is sensed by a pressure or other form of transducer to produce an electrical signal whose frequency is proportional to flow rate.
A typical vortex-shedding flowmeter of known design includes a flow tube having mounting flanges at the ends thereof. One mounting flange is bolted or otherwise attached to the complementary flange on an upstream pipe section of the pipeline carrying the fluid whose rate is to be metered, the other mounting flange being attached to the complementary flange of the downstream pipe section. A conventional vortex-shedding flowmeter has a fixed metering range within which it is capable of accurately measuring flow rate. This range is largely determined by linearity requirements, signal recovery parameters and internal velocity limitations.
In some instances, it becomes necessary to accurately measure low flow velocities, which lies below the normal operating range of a standard vortex-type meter. With existing meters, it is not possible, in the field, to alter the operating range of an installed meter. Moreover, it is difficult to produce a small capacity flowmeter, for with existing vortex-shedding meter structures, the vortex-sensing system cannot economically be miniaturized.